1. Field of Invention
The present invention relates to the optical networking field, and in particular to an optical device and a dispersion compensator using the device.
2. BACKGROUND
Chromatic dispersion compensation may be necessary for optical networks; and tunability may be the key for reconfigurable or agile networks. Current tunable methods typically cannot compensate the chromatic dispersion slope of the optical fiber over a specific wavelength range (e.g. C band), which is an important problem for DWDM systems, though the amount of chromatic dispersion can be tuned.
Conventional chromatic dispersion compensators typically use specially designed optical fibers, which are expensive, and can not be tuned. Next generation optical networks may need cheaper and agile devices, and thus tunability or reconfigurability are desirable. Tunable dispersion compensation is typically one of the blocking factors for realizing agility of next generation optical networks. Some tunable dispersion compensating modules (DCMs) e.g. Fiber Bragg Grating (FBG) and etalon based DCMs are developed but they may not be satisfactory since they are not capable of compensating the dispersion slope of the fiber used for telecommunication (see Reference 1, Christopher R. Doerr, Optical compensation of system impairments, 5-10 Mar. 2006, OFC 2006).
FIG. 1 shows an optical pulse before entering optical fiber 101 (100), after exiting optical fiber 101 and before entering dispersion compensator 102 (120), and after exiting dispersion compensator 102 (140). An optical pulse consists of different chromatic components. When traveling through a medium e.g. a single mode optical fiber (referring to 101 in FIG. 1), the different components of light have different speeds, so these chromatic components get dispersed after traveling a distance, and the extent of the dispersion is proportional to the traveling distance of the medium (referring to 120 in FIG. 1). The dispersion of different chromatic components causes “distortion” of the shape of the optical pulse thus degrades the transmission performance of the digital networks. Compensation of the chromatic dispersion can help restore the “distorted” optical pulse thus improving the transmission performance of the digital networks.
Future optical networks may need reconfigurability, this is because the traveling distance needed for the optical pulse may vary with different network configurations, as a result the amount of the dispersion to be compensated should also be reconfigurable, or, tunable.
Particularly, FIG. 2 is an exemplary schematic diagram showing the usage of Tunable Chromatic Dispersion Compensator (TCDC) in an agile optical network. As shown in FIG. 2, when changing network transmission configuration from a first configuration A to B to a second configuration A to C by the optical router 201, transmission distance also changes from A-B to A-C, in this case the Tunable Chromatic Dispersion Compensators (TCDCs) 202 are needed to perform reconfigurability.
FIG. 3 shows an exemplary schematic overall diagram of the Tunable Chromatic Dispersion Compensator (TCDC) 202. The single channel TCDC 202 mainly consists of a dispersive grating 301, a telescope structure (302 and 303), a single mode waveguide 304 and a tangential coupling grating 305. The dispersive grating 301 disperses the light to a small angle, the telescope structure, which consists of an object lens 302 and an eye piece 303, can magnify this angle by a factor of fo/fe, where fo and fe are the focal lengths of the object lens and eye piece 303 respectively. Light exiting the eye piece 303 will be coupled to the arc single mode waveguide 304 by the arc tangential coupling grating 305, thus different colors experience different delay after exiting the pigtail, so dispersion is compensated. If the radius of the arc is r, dispersed angle is 2θ, then the maximum dispersion distance able to be compensated is 2rθ.
As discussed before, the “maximum compensated dispersion distance” is:
      2    ⁢    r    ⁢                  ⁢    θ    =      2    ⁢    r    ×          arctan      ⁡              (                              tan            ⁡                          (              α              )                                ⁢                                    f              o                                      f              e                                      )            
The amount of the compensated dispersion can be tuned by changing the magnification factor of the telescope structure (302 and 303) as shown in FIG. 3. The focal length of the object lens 302 is fixed in this solution, so a deformable mirror with a variable focal length can be used for the eye piece 303. A kind of deformable mirror using piezo-electric actuators is proposed in a publication by Chris R. Doerr (see Reference 2, C. R. Doerr, et. al., 40-Gb/s colorless tunable dispersion compensator with 1000-ps/nm tuning range employing a planar lightwave circuit and a deformable mirror, 6-10 Mar. 2005, OFC 2005).
The present invention is intended to overcome at least some the above problems for “tunable DCMs” for future dynamically reconfigurable optical networks.